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Topological Properties of Titanate Nanotubes Tao Gao,* Qinglan Wu, Helmer Fjellvåg, and Poul Norby Department of Chemistry and Center for Materials Science and Nanotechnology, UniVersity of Oslo, P.O. Box 1033, N-0315 Oslo, Norway ReceiVed: January 24, 2008; ReVised Manuscript ReceiVed: April 2, 2008
We report a simple ultrasonic-assisted ion-exchange/intercalation process that enables unwrapping onedimensional (1D) titanate nanotubes into two-dimensional (2D) titanate nanosheets. The existence of this 1D to 2D topotactic transformation reveals that the titanate nanotubes could be considered as quasi-2D crystallites with excellent 2D properties such as exfoliation/delamination reactivities although they have the 1D morphology. The resulting titanate nanosheets possess larger band gap energy (∼3.75 eV) than that of the original nanotubes (∼3.30 eV), which might be attributed to the quantum size effect within the 2D titanate nanosheets with small thickness. Introduction nanotubes,1
significant efforts Since the discovery of carbon also have been devoted to preparation and characterization of inorganic nanotubes of more chemical complexity.2 Among such materials, TiO2-based nanotubes are of particular interest because of their potential applications as photocatalysts,3 sensors,4 electrochemical capacitors,5 and as lithium-inserting materials.6 To date, TiO2-based nanotubes with different microstructures have been prepared by various techniques, such as template method,7 anodic oxidation,8 and wet chemical method.9–12 The wet chemical method, based on a hydrothermal treatment of TiO2 in concentrated aqueous solution of NaOH, is fairly simple and enables the production of pure titanate nanotubes at relatively low temperatures. It is worth noticing that the nanotubes obtained by the NaOH treatment of TiO2 followed by a thorough acid washing are protonic titanates10–12 rather than crystalline TiO2.9 However, the exact crystalline nature of the titanate nanotubes is currently disputed.10–12 Structurally, the nanotubes produced via the alkaline treatment of TiO2 are open-ended nanoscrolls with spiral cross-sections,9–12 which are isomorphous to those formed by scrolling lamellar precursors, such as VOx,2a,13 Ge/Si,14 WS2,15 K4Nb6O17,16 H2La2Ti3O10,17 and MnO2 nanotubes.18 It is believed that these scroll-like nanotubes are different from the cylinder-like nanotubes with concentric circle cross-sections, such as carbon nanotubes.1 For example, these nanoscrolls might have much larger topological flexibility than that of the cylinder-like nanotubes because they are composed of layers that may have a considerable expansion when unscrolled.2a In this regard, it is worth arguing whether or not the titanate nanotubes adopt one-dimensional (1D) properties.19 However, so far no experimental data concerning the topological properties of the titanate nanotubes have been reported. In this paper, we report a simple soft chemistry method that enables unwrapping 1D scroll-like titanate nanotubes into twodimensional (2D) titanate nanosheets. This work contributes to the understanding of interlayer chemistry, topological property, and structural characteristics of the titanate nanotubes from a new point of view. It demonstrates that the titanate nanotubes * To whom correspondence should be addressed. E-mail: tao.gao@ kjemi.uio.no.
adopt excellent 2D properties that are similar to those of the bulk-layered titanates such as Na2Ti3O720 and lepidocrocite-type titanates,21 although they have the 1D morphology. Moreover, it suggests that the titanate nanotubes were formed by rolling up the lepidocrocite-type titanate nanosheets12 rather than the monoclinic trititanate ones.10 Experimental Procedures Reagents and Materials. Anatase TiO2 nanopowders (particle size, < 25 nm; purity, 99.7%), tetrabutylammonium ((C4H9)4NOH, abbreviated as TBAOH hereafter) hydroxide aqueous solution (40 wt %), and polyethylenimine aqueous solution were purchased from Sigma-Aldrich Co. and used as received. Double distilled (Type II) water was used throughout the experiment. Synthesis of Titanate Nanotubes. Titanate nanotubes used in this work were synthesized by using the procedures developed by Kasuga et al.9a with a small modification. TiO2 nanopowder (0.1 g) was added in 100 mL NaOH aqueous solution (10 M). After stirring at room temperature for 30 min, the resulting white suspension was charged into Teflon-lined autoclaves and heated to 140 °C for 24-72 h. The product was acid-washed, which involved stirring the sample in 0.2 M HNO3 aqueous solution for three days. The acid solution was renewed every 24 h to promote a complete ion exchange. The material was then filtered, washed with water, and dried at 80 °C overnight to give the as-prepared titanate nanotubes. Unwrapping of Titanate Nanotubes. Fifty milligrams of the as-prepared titanate nanotubes was ultrasonically dispersed in 50 mL of 10 wt % TBAOH aqueous solution. After being ultrasonically treated for 1 h, the obtained suspension was stirred at 50 °C for 3-7 days to promote the ion-exchange/intercalation reaction. The resulting suspension was allowed to stand for 1 week. Thereafter, the top 90% of the suspension in the flask was removed and used for further experiments. The remaining 10% of the suspension in the flask, which contains unreacted titanate nanotubes, was treated further by adding fresh organic solution and repeating the ion-exchange/intercalation process to achieve a complete reaction. Characterizations. X-ray powder diffraction (XRD, Siemens D5000 powder diffractometer, with Cu KR1 radiation), fieldemission scanning electron microscopy (FE-SEM, FEI Quanta
10.1021/jp800714s CCC: $40.75 2008 American Chemical Society Published on Web 05/16/2008
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200F), transmission electron microscopy (TEM, Philips CM30ST), UV-visible optical absorption (Cary 100 Bio UV-visible spectrophotometer), and Raman scattering spectroscopy were used to characterize the as-synthesized materials. SEM samples of titanate nanosheets were prepared by immersing the pretreated silicon substrates (treated with a 5 g/L polyethylenimine solution to introduce positive charges onto the surfaces) into the titanate nanosheet suspension for 10 min to deposit the titanate nanosheets. TEM samples of titanate nanosheets were prepared by dropping a small amount of the diluted titanate nanosheet suspension on the TEM grids and drying in air before characterization. Samples for XRD and optical absorption were prepared by centrifugating the titanate nanosheet suspension at 10 000 rpm for 30 min: the precipitate was cast directly on glass substrates for XRD analysis; the resulting colloidal solution was for optical absorption measurement. The samples for Raman scattering spectra were illuminated by using a 632.8 nm laser with a 10× objective on an Olympus BX 40 confocal microscope. All experiments were performed at room temperature. Results and Discussions Figure 1a shows a typical XRD pattern of the as-prepared titanate nanotubes. The XRD pattern is basically the same as those reported previously for the nanotubes prepared via the wet chemical method,9–12 suggesting that the identical nanotubular structure were obtained in this work. Note that all diffraction peaks in Figure 1a can be readily indexed based on an orthorhombic lepidocrocitetype titanate H0.7Ti1.82500.175O4 · H2O (0 ) vacancy) with lattice parameters of about a ) 0.378 nm, b ) 1.873 nm, and c ) 0.297 nm;12a the diffraction peaks at 2θ ) 10.2, 24.6, 28.7, and 48.5° correspond with (020), (110), (130), and (200) reflections of the lepidocrocite-type titanate H0.7Ti1.82500.175O4 · H2O. The interlayer distance of the as-prepared titanate nanotubes, estimated from the XRD data, is about 0.86 nm. Figure 1b is an SEM image of the as-prepared titanate nanotubes in which high purity wirelike 1D nanostructures are entangled together. The detailed morphology of the titanate nanotubes was characterized by TEM, as shown in Figure 1c. It reveals that the as-prepared titanate nanotubes are typically 8-12 nm in diameter and several hundreds of nanometers in length. The measured interlayer distance of the titanate nanotubes is about 0.8 nm (inset of Figure 1c), which is in agreement with the XRD data. The titanate nanotubes are known to be ion exchangeable.10–12 We have extended our investigations to the interlayer chemistry and the topological property of the titanate nanotubes by intercalating the bulky organic molecule into the interlayer regions of the nanotubes. As expected, a major structural rearrangement takes place after the titanate nanotubes were treated with the TBAOH aqueous solution, which is evidenced by the XRD pattern shown in Figure 2a. The reflections at d ≈ 1.883, 0.942, and 0.621 nm can be indexed as the (001), (002), and (003) reflections of a lamellar structure with an interlayer distance of about 1.88 nm. The increased interlayer distance of 1.88 nm compared to 0.86 nm for the as-prepared titanate nanotubes indicates that the bulky TBA+ ions (0.95-1.05 nm in diameter)22 have been introduced into the interlayer region of the titanate nanotubes. The well-defined (001) reflections are supposed to result from flat multilayers rather than from the tubular frameworks, as shown schematically inset of Figure 2a. It suggests that the nanotubes might be unwrapped into lamellar nanosheets due to the intercalation of bulky TBA+ ions. This possible morphological transformation was analyzed by SEM. Figure 2b shows clearly that 2D sheetlike materials are produced after the original wirelike 1D nanostructures (Figure 1b) were
Figure 1. (a) XRD pattern, (b) SEM and (c) TEM images of the asprepared titanate nanotubes. Indexes given in panel a are based on the orthorhombic lepidocrocite-type titanates. Inset of panel c shows an enlarged TEM picture of a triple-layered nanotube with an interlayer distance of about 0.8 nm.
treated with the TBAOH. TEM observations (Figure 2c) reveal further that the nanotubular structures of the as-prepared titanate nanotubes (Figure 1c) are transformed into lamellar solids (Figure 2c). Note that an ∼100% conversion can be achieved by optimizing the experimental conditions, revealing that these lamellar solids are resulted from the titanate nanotubes.23
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Figure 3. SEM images of the unwrapped titanate nanosheets (a-d) with plate-like morphology and some intermediates (e) produced during the ion-exchange/intercalation process. Inset of (a) shows a representative nanosheet.
Figure 2. (a) XRD pattern, (b) SEM and (c) TEM images of the titanate nanotubes after treated with TBAOH. Inset of panel a shows a structure model to interpret the observed XRD peaks at low angles; the light grey layer and the dark grey ball represent the titanate nanosheets and TBA+ ions, respectively.
The morphological transformation from 1D scroll-like titanate nanotubes to 2D lamellar titanate nanosheets were characterized further by detailed SEM analyses (Figure 3). It is found that the individual sheetlike materials are usually several hundreds
of nanometers in width and up to 1 µm in length. The inset of Figure 3a shows a representative rectangle-like nanosheet with length of about 600 nm and width of about 200 nm. Some nanosheets have irregular shapes and small lateral sizes, which might be due to the breaking down of the large nanosheets during the unwrapping process. It is difficult to obtain an accurate size distribution of the resulting nanosheets. The edges of the resulting nanosheets are usually curved along the width direction (see for example, Figure 3c,d), which is different remarkably from the sheetlike byproduct of the titanate nanotubes9–12 and demonstrates clearly that they are resulted
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SCHEME 1: Irreversible Topotactic Transformation from Titanate Nanotubes to Titanate Nanosheets
Figure 4. Normalized optical absorption spectra of the titanate nanotubes and the resulting titanate nanosheets. Inset shows the (ηhV)1/2 vs hV plot (η, absorption coefficient; hV, photon energy).
from the scroll-like precursors. During the SEM characterizations (Figure 3e), some half-tube and half-sheet structures and sheets with curved edges15–17 are also observed, which are supposed to represent intermediates formed during the unwrapping process. The above experimental results demonstrate the existence of the topotactic transformation from 1D scroll-like titanate nanotubes to 2D lamellar titanate nanosheets. In principle, the nanotubes built up by scrolling lamellar precursors13–18 can be unwrapped into individual layers only if proper conditions are offered. In case of the titanate nanotubes, we have demonstrated that it can be performed by intercalating bulky organic amine (such as TBA+ ions) into the interlayer regions of the nanotubes. Note that this process is different from that reported by Saupe et al. in their K4Nb6O17 nanotubes,16b in which ionic strength of the solution was proposed to be a dominative parameter. We suggest that thanks to the electrostatic interaction of negatively charged titanate nanosheets and positively charged amine ions with rather large sizes20,22 the ion-exchange/intercalation reaction will at the same time swell and unwrap the nanotubes into nanosheets. This process is schematically illustrated in Scheme 1. It should be pointed out that ultrasonic treatment is important to achieve the unwrapping of the titanate nanotubes. It is possible that the somewhat extreme conditions generated by the sonication24 might supply sufficient energy to open up the outermost layer of the nanotubes to initiate the ion-exchange/intercalation. Thereafter, the reaction of the exchanged H+ ions with OH- ions in the exfoliating solution will further promote the ion-exchange/intercalation to swell and unwrap the nanotubes into nanosheets. Interestingly, the topotactic transformation from the 1D scrolllike titanate nanotubes to the 2D lamellar titanate nanosheets (Scheme 1) is found to be irreversible, which is different from that observed in the formation of the K4Nb6O17 nanotubes.16b The irreversibility of the unwrapping process reveals clearly that the formation of the titanate nanotubes might proceed via not a simple rolling process as proposed in ref 18 but a complex self-assembly process in concentrated hot alkali solutions.10d On the other hand, the irreversibility of the topotactic transformation may correlate with the morphology/size of the unrolled titanate nanosheets. For example, the unrolled tails of the titanate nanotubes given by the intercalation of the TBA+ ions may be cut off during the ultrasonic radiation;25 if the unscrolled broken nanosheets are too small to be scrolled again, they will keep
the unscrolled morphology. Formation of narrow nanosheets shown in Figure 3c seems in harmony with this assumption. As evidenced by this topotactic transformation (Scheme 1), the titanate nanotubes possess excellent 2D properties such as exfoliation/delamination reactivities. Normally, these 2D properties are appeared in bulk-layered materials, such as K4Nb6O17,16,25b birnessite-type MnO2,18,22 layered titanates,20,21,25a,26 and layered double hydroxides.27 In this regard, the existence of the topotactic transformation may suggest that the titanate nanotubes could be considered as quasi-2D crystallites although they have the 1D morphology. Note that the titanate nanotubes are essentially layered materials because the walls of the nanotubes are usually composed of more than one layers (Figure 1c). This hypothesis can be tested by considering property changes of the titanate nanotubes during the topotactic transformation. Figure 4 shows UV-vis absorption spectra of the as-prepared titanate nanotubes and the resulting titanate nanosheets for comparison. To estimate the optical band gap energy Eg, the square root of absorption coefficient times photon energy (ηhV) is plotted against the incident photon energy (hV).26 This plot gives a straight line in a photon energy range close to the absorption threshold, as shown inset of Figure 4. The band gap energy obtained for the titanate nanosheets is about 3.75 eV, which is larger than that of the titanate nanotubes, 3.30 eV. This band gap-energy shift seems to correlate with the topotactic transformation of the titanate nanotubes and can be attributed to the quantum size effect within the 2D titanate nanosheets with small thickness of ∼1 nm.22,26 It is known that the blue shift in optical absorption of quasi-2D crystallites is governed predominantly by their layer thickness.26 Consequently, the band gap energy of the titanate nanotubes will be dominated by their wall thickness rather than their diameters.19 It is important to point out that the unwrapping process (Scheme 1) does not change the microstructures of the titanate nanosheets that form the walls of the original titanate nanotubes. In this regard, the topotactic transformation reported here offers an excellent opportunity to understand the crystalline structures of the titanate nanotubes from a new point of view. Figure 5 shows the Raman scattering spectra of the titanate nanotubes and the resulting titanate nanosheets for comparison. The Raman scattering spectrum of the resulting titanate nanosheets is basically the same as that of the titanate nanotubes, except for some slight band shifts. The Raman band shifts correlate to the corresponding Ti-O bond changes, such as the changes of bond
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Figure 5. Raman scattering spectra of the titanate nanotubes (a) and the resulting titanate nanosheets (b). The curves are shifted vertically for clarity.
lengths and/or bond angles, when the scroll-like nanotubes (Figure 5a) are unwrapped into flat nanosheets (Figure 5b). Because the polymerization manner of the TiO6 octahedra does not change during the scrolling/unwrapping process, only certain Raman band shifts can be expected. Note that the presence of the three Raman bands at 278, 454, and 680 cm-1 in the titanate nanosheets (Figure 5b) is indicative of a well-developed 2D lepidocrocite-type layered structure.28 It reveals clearly that the titanate nanotubes are formed by rolling up the lepidocrocitetype titanate nanosheets12 rather than the monoclinic trititanate ones.10 A Raman scattering study on the crystalline structure of the titanate nanotubes will be reported in a forthcoming paper. Conclusions In summary, the topological property of the titanate nanotubes prepared via an alkaline treatment of crystalline TiO2 has been studied via a soft chemical method that enables unwrapping the 1D scroll-like nanotubes into 2D lamellar nanosheets. The topotactic transformation suggests that the titanate nanotubes could be considered as quasi-2D crystallites with excellent 2D properties such as exfoliation/delamination reactivities, although they possess the 1D morphology. The unwrapping process offers also the possibility to study the relationship between the titanate nanotubes and their corresponding building blocks, the titanate nanosheets, which are of particular interest in the structural studies of the titanate nanotubes. By further optimizing the experimental procedures, the method used here could be extended to study the interlayer chemistry and topological property of other nanotubes/nanoscrolls with more chemical complexity. Acknowledgment. We thank Dr. Frank Krumeich and Professor Reinhard Nesper (Laboratory of Inorganic Chemistry, ETH Zurich) for their help on TEM analysis. We are grateful to the valuable comments from the reviewers. The authors would like to acknowledge financial assistance from the Research Council of Norway through the NANOMAT program (163565431).
(1) Iijima, S. Nature 1991, 354, 56. (2) (a) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (b) Xiong, Y.; Mayers, B. T.; Xia, Y. Chem. Commun. 2005, 5013. (c) Rao, C. N. R.; Nath, M. Dalton Trans. 2003, 1. (3) Adachi, M.; Murata, Y.; Harada, M.; Yoshikawa, S. Chem. Lett. 2000, 942. (4) (a) Liu, S.; Chen, A. Langmuir 2005, 21, 8409. (b) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624. (5) Wang, Y. G.; Zhang, X. G. J. Electrochem. Soc. 2005, 152, A671. (6) (a) Armstrong, G.; Armstrong, A. R.; Canales, J.; Bruce, P. G. Chem. Commun. 2005, 2454. (b) Zhang, H.; Li, G. R.; An, L. P.; Yan, T. Y.; Gao, X. P.; Zhu, H. Y. J. Phys. Chem. C 2007, 111, 6143. (7) Imai, H.; Takei, Y.; Shimizu, K.; Matsuda, M.; Hirashima, H. J. Mater. Chem. 1999, 9, 2971. (8) (a) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331. (b) Macak, J. M.; Tsuchiya, H.; Taveira, L.; Aldabergerova, S.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 7463. (9) (a) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. AdV. Mater. 1999, 11, 1307. (b) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yang, Z. Y.; Wang, N. Appl. Phys. Lett. 2003, 82, 281. (10) (a) Chen, Q.; Zhou, W.; Du, G.; Peng, L. M. AdV. Mater. 2002, 14, 1208. (b) Chen, Q.; Du, G. H.; Zhang, S.; Peng, L. M. Acta Crystallogr. 2002, B58, 587. (c) Sun, X.; Li, Y. Chem.sEur. J. 2003, 9, 2229. (d) Zhang, S.; Chen, Q.; Peng, L. -M. Phys. ReV. B 2005, 71, 014104. (11) (a) Yang, J.; Jin, Z.; Wang, X.; Li, W.; Zhang, J.; Zhang, S.; Guo, X.; Zhang, Z. Dalton Trans. 2003, 3898. (b) Tsai, C. C.; Teng, H. Chem. Mater. 2006, 18, 367. (12) (a) Ma, R.; Bando, Y.; Sasaki, T. Chem. Phys. Lett. 2003, 380, 577. (b) Ma, R.; Sasaki, T.; Bando, Y. Chem. Commun. 2005, 948. (c) Ma, R.; Fukuda, K.; Sasaki, T.; Osada, M.; Bando, Y. J. Phys. Chem. B 2005, 109, 6210. (13) (a) Krumeich, F.; Muhr, H. -J.; Niederberger, M.; Bieri, F.; Schnyder, B.; Nesper, R. J. Am. Chem. Soc. 1999, 121, 8324. (b) Niederberger, M.; Muhr, H. -J.; Krumeich, F.; Bieri, F.; Gu¨nther, D.; Nesper, R. Chem. Mater. 2000, 12, 1995. (14) Schmidt, O. G.; Eberl, K. Nature 2001, 410, 168. (15) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am. Chem. Soc. 2002, 124, 1411. (16) (a) Du, G.; Chen, Q.; Yu, Y.; Zhang, S.; Zhou, W.; Peng, L. M. J. Mater. Chem. 2004, 14, 1437. (b) Saupe, G. B.; Waraksa, C. C.; Kim, H. N.; Han, Y. J.; Kaschak, D. M.; Skinner, D. M.; Mallouk, T. E. Chem. Mater. 2000, 12, 1556. (17) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2000, 12, 3427. (18) Ma, R.; Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115. (19) Bavykin, D. V.; Gordeev, S. N.; Moskalenko, A. V.; Lapkin, A. A.; Walsh, F. C. J. Phys. Chem. B 2005, 109, 8565. (20) (a) Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Mater. Chem. 2004, 14, 165. (b) Kaito, R.; Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Mater. Chem. 2002, 12, 3463. (21) (a) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2004, 126, 5851. (b) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682. (c) Sasaki, T.; Nakano, S.; Yamauchi, S.; Watanabe, M. Chem. Mater. 1997, 9, 602. (22) Gao, Q.; Giraldo, O.; Tong, W.; Suib, S. L. Chem. Mater. 2001, 13, 778. (23) There are small amounts (less than 5%) of sheetlike by-products coexisting with the titanate nanotubes. (24) (a) Suslick, K. S. Science 1990, 247, 1439. (b) Gao, T.; Wang, T. Chem. Commun. 2004, 2558. (25) (a) Tanaka, T.; Fukuda, K.; Ebina, Y.; Takada, K.; Sasaki, T. AdV. Mater. 2004, 16, 872. (b) Miyamoto, N.; Nakato, T. J. Phys. Chem. B 2004, 108, 6152. (26) (a) Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997, 101, 10159. (b) Sato, H.; Ono, K.; Sasaki, T.; Yamagishi, A. J. Phys. Chem. B 2003, 107, 9824. (27) Wu, Q.; Olafsen, A.; Vistad, Ø. B.; Roots, J.; Norby, P. J. Mater. Chem 2005, 15, 4695. (28) Gao, T.; Fjellvåg, H.; Norby, P. University of Oslo, Oslo, Norway. Unpublished data, 2008.
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